This invention relates to the etching of copper, and more particularly to novel compositions and methods useful in the etching of copper most particularly for the preparation of printed circuit boards.
In the preparation of printed circuit boards copper foil is clad to a flat baseboard or substrate of high dielectric strength, commonly referred to as the "insulator". By subtractive techniques, copper is removed from the foil in a pattern complementary to the pattern of the circuit to be produced. According to common practice the copper foil is coated with a photoresist (or "etch resist") whose solubility or dispersibility in a developing solution is changed when the resist is struck by light or other electromagnetic energy. Positive working resists are initially impervious to the developing agent, but are rendered removable thereby upon exposure to light. Negative working resists are initially soluble but are rendered impervious by exposure to light or other electromagnetic energy.
In preparing a circuit board, the resist is applied over the foil. The photoresist layer is exposed to light through a photographic transparency or stencil having a pattern therein corresponding to the circuit to be printed. Thereafter, the photoresist is contacted by the developer and removed from the foil in a pattern complementary to the circuit configuration. The foil in this complementary pattern is then contacted with an etching solution for removal of copper.
Solutions commonly employed for etching copper are alkaline or ammoniacal solutions, copper chloride solutions or ferric chloride solutions. In recent years, however, there has been a shift toward the use of etching solutions which comprise mixtures of hydrogen peroxide and sulfuric acid. The advantages of the peroxide sulfuric acid system include competitively low cost, ease of use, and in situ recovery of copper from the used etching solution in the form of copper sulfate or copper metal. Moreover, the solution from which copper has been recovered can be regenerated by replenishment of the hydrogen peroxide therein. This procedure provides not only for recovery of valuable copper but minimizes the environmental burden otherwise arising from the disposition of used etching solution.
However, the productivity of the peroxide/sulfuric acid etching process is often limited, sometimes severely, by the exhaustion of hydrogen peroxide from the etching solution. Such exhaustion arises not only from consumption in the oxidation of copper, but also from decomposition of unstable hydrogen peroxide. In particular, it has been found that instability of hydrogen peroxide increases with the concentration of heavy metals such as copper, as a consequence of which the rate of exhaustion accelerates progressively with repeated use of the etching solution and buildup of the copper content thereof. Thus the etch rate tends to fall off drastically, requiring frequent regeneration of the etching solution to avoid severe loss of productivity.
In an effort to stabilize acid/peroxide etching solutions against hydrogen peroxide decomposition, various stabilizing agents have been proposed. Thus Solenberger et al., U.S. Pat. No. 3,801,512 describes an acid/peroxide copper etching solution which contains a stabilizer such as an aryl sulfonic acid, N,N-lower alkyl aniline, sulfamic acid, sulfolane, sulfolene, di-normal lower alkyl sulfones, di-normal lower alkyl sulfoxides and the like. However, while such stabilizers are effective for retarding the decomposition of the hydrogen peroxide, they tend to impede the etch rate at a given hydrogen peroxide level. Moreover, where the etching bath is used repetitively in a commercial etching process, deterioration in the etching rate becomes progressive even in the presence of a stabilizer of the type used by Solenberger et al.
Dutkewych et al. Pat. No. 4,130,454 describes a copper etching solution comprising sulfuric acid, hydrogen peroxide, an alkyl or aryl sulfonic acid, and a sodium molybdate. The molybdenum compound serves as a co-oxidant which is said to coact synergistically with the hydrogen peroxide to exalt the etch rate.
Banush et al. Pat. No. 3,668,131 describes a peroxide/sulfuric acid etching system which uses a urea or thiourea additive to increase the capacity and rate of etching. According to the Banush et al. disclosure, solutions containing such additives as methylurea, ethylurea, p-chlorophenylurea, and the like, etch copper at high rates and with high capacity, and also exhibit good storage life without substantial deterioration over periods of 4 to 10 days after preparation or longer.
Achenbach Pat. No. 3,463,733 describes copper etching baths comprised of sulfuric acid, hydrogen peroxide, mercuric chloride, and a stabilizer for the hydrogen peroxide which may be selected from among urea, semicarbazide, biuret, barbituric acid, and dipropylbarbituric acid.